Studies on Multilevel Twenty-Four Sided Polygonal Voltage Space Vector Structure Generation With a Single DC Link for Variable Speed Drive Applications

Abstract

Multilevel voltage source inverters have become a widely accepted and cost-effective power converter technology for applications requiring high-power medium-voltage control. The demand of power level requirement has reached operational limits of megawatt range. Multilevel inverters (MLI) find applications in power transmission and distribution systems like HVDC which are connected with high voltage network lines and controlled ac drives operating at medium voltage levels. For low voltage applications, most prevalent topology which dominates industrial drives is conventional two-level inverter. With state of the art semiconductor technology, self-commutating converters with arrangement of several low voltage devices, help achieving voltage ranges till hundreds of kilovolts. Apart from high voltage operational capability, advantages like power quality control, better electromagnetic compatibility, lower switching losses, keep multilevel inverters a class above the conventional two-level inverters. In order to attain good waveform quality, the inverter needs to switch at very high frequencies. The harmonics appear only at switching frequency sidebands, which can be easily filtered externally. But, considering large voltage stress handled by the devices in two-level inverter and large switching loss in the devices degrade the efficiency of system substantially. Specific to applications like medium voltage drives, the major issues on electromagnetic interference, device stress, harmonic performance, and dv/dt control are mostly addressed by employing multilevel inverters. Most popular multilevel inverter topologies are neutral-point clamped inverters, flying capacitor inverters, and cascaded H-bridge inverters. These basic MLIs are further used to obtain hybrid multilevel inverters generating more number of voltage levels. Other applications of multilevel inverters include photovoltaic, hydel and wind energy systems, energy storage and management systems, electric vehicle applications, traction drives etc. As a 24-sided polygon is closer to a circle than a hexagon or a 12-sided polygon, the above presented schemes generate high quality motor phase voltage waveforms without using any external filters. A physical sine-wave filter can be completely relaxed for such variable speed drive applications, and the dynamic performance is never compromised since the filtering action is performed by switched capacitors. The topologies and modulation techniques presented are optimized for low switching frequency operation of large voltage blocking inverters and shifting relatively higher frequency switchings to low voltage cascaded H-Bridge inverters. Above all, single DC source operation can bring down the cost and complexity of the system drastically enabling easier back to back operation for drive. Also, such schemes can be directly driven from battery operated systems in electric vehicles without any passive sine filters. With all the mentioned advantages, the proposed drive schemes are highly suitable for high performance, medium voltage drive applications.